The invention relates generally to the field of optical modulation and, in particular, to methods and apparatus for high-speed external optical modulations.
Optical modulators impress or modulate RF (or microwave) electrical signals onto a light beam in order to generate a modulated optical beam that carries data. Modulators either directly modulate the optical beam as it is generated at the optical source or externally modulate the optical beam after it has been generated. Direct modulation is typically accomplished by modulating the drive current of the optical source. An integrated electro-absorptive modulator can modulate the optical intensity of light leaving the source as well.
External modulation can be accomplished by using an external modulator that is separate from the optical source. External modulation is advantageous because it can modulate signals over a very wide bandwidth. External modulators are typically voltage-controlled devices that include a traveling-wave electrode structure, which is positioned in close proximity to the optical waveguide. The electrode structure produces an electric field that overlaps the optical waveguide over a predetermined distance (the interaction length) and causes an electromagnetic interaction which modulates the optical signal.
Lithium niobate (LN) electro-optic external modulators are increasingly being used to modulate data on optical signals that are being transmitted at very high data rates and over long distances. Lithium niobate modulators are advantageous because they can modulate optical signal over a broad frequency range, they modulate optical signals with minimal optical frequency shift (frequency xe2x80x9cchirpxe2x80x9d), and they operate over a broad wavelength range. These features are particularly desirable for Dense Wavelength Division Multiplexing (DWDM) broadband optical communication systems that transmit optical signals with many optical wavelengths through a single optical fiber.
Modulators used for transmission at high speeds and over long distances must be efficient to avoid the use of expensive electronic amplifiers and digital drivers. In addition, modulators need to be compact in order to minimize the required space on the transmitter card.
Lithium niobate crystals have an inherent mismatch between the velocity of optical and electrical signals propagating through the crystal which impacts modulation efficiency. The RF propagation index is significantly higher than the optical refractive index of lithium niobate. That is, the lithium niobate crystal slows the RF signal relative to the optical signal so that it takes the RF signal a longer period of time to travel over the interaction distance. Thus, the RF signal becomes out-of-phase with or xe2x80x9cwalks offxe2x80x9d the optical signal. Consequently, the modulation becomes inefficient. The longer the interaction distance, the greater the inefficiency. Using a buffer layer can minimize velocity walk-off, however, the required interaction length is long.
FIG. 1 illustrates a top view of a prior art electro-optic device 10 that compensates for the velocity mismatch between the optical and electrical signals propagating through the device by using phase reversal sections that are co-linear with the optical waveguide. The device 10 includes an optical waveguide 12 and RF electrodes 14 that are positioned in zero degree phase sections 16 and in phase reversal sections 18. The phase reversal sections 18 periodically flip the RF electrodes 14 to either side of the optical waveguide 12 to produce a 180 degree phase shift in the RF signal relative to the optical signal. The RF electrodes 14 are positioned to alternate between the zero degree phase shift sections 16 and the 180 degree phase shift sections 18. The length of the zero degree phase shift sections 16 is chosen so that the RF signal xe2x80x9cwalks offxe2x80x9d the optical signal approximately 180 degrees before it is flipped 180 degrees in the phase reversal sections 18.
FIG. 2 illustrates a top view of a prior art electro-optic device 30 that compensates for the velocity mismatch between the optical and electrical signals propagating through the device 30 by using co-linear but intermittent interaction sections. The device 30 includes an optical waveguide 32 and RF electrodes 34 that are positioned to alternate between an interaction region 36 and a non-interaction region 38 relative to the optical waveguide 32. The length of the interaction region 36 is chosen so that the RF signal xe2x80x9cwalks offxe2x80x9d the optical signal by as much as 180 degrees of phase shift before it is routed away from the optical waveguide 32 in a co-linear direction and into the non-interaction region 38. The length of the non-interaction region 38 is chosen so that the RF signal becomes phase matched with the optical signal at the end of the non-interaction region 38.
One disadvantage of prior art electro-optic devices that compensate for the velocity mismatch between the optical and electrical signals propagating through the device is that they have relatively low modulation efficiency per unit length. This is because the phase of the RF signal is modified with co-linear sections that are positioned at intervals of 180 degrees. When the difference in phase between the RF and optical signals approaches 180 degrees, the incremental increase in modulation depth with incremental change in electrode length approaches zero. Therefore, the total length of the device must be increased in order to achieve the required modulation. Increasing the length of a lithium niobate device increases the size of the package containing it, which is undesirable, because of the limited space on the transmitter board. State-of-the-art DWDM systems have stringent space requirements due to their high channel count. In addition, more expensive and larger power supplies must be used because higher drive voltages are required.
It is therefore a principal object of this invention to provide an electro-optic device that includes a compensation network that modifies at least one of the phase or the amplitude of the electrical signal relative to the phase or amplitude of the accumulated modulation on the optical signal without introducing significant loss or decreasing the modulation efficiency. It is another principle object for such a compensation network to compensate for velocity mismatch between the electrical signal and the optical signal. It is another principle object for such a compensation network to compensate for the effects of external perturbations in the substrate of the modulator, such as the effects of temperature on a lithium niobate substrate. It is another principle object for such a compensation network to be removably attached to the device to facilitate modifying the frequency response of the device. It is yet another principle object of the present invention to construct a modulator with such a compensation network that is used in conjunction with prior art broadband modulator to form a combined modulator that is capable of producing bandwidth extension of the broadband modulator into the narrow band modulator region.
A principal discovery of the present invention is that an electro-optic device can be constructed with a compensation network that temporarily directs the electrical signal in a path that is in a non-co-linear direction relative to the direction of propagation of the optical signal and that such a compensation network has numerous advantages over the prior art. For example, such a compensation network can modify the phase of the electrical signal relative to the optical signal in order to minimize the effects of velocity mismatch, while introducing very low loss. Such a compensation network can also compensate for the effects of external perturbations on the electro-optic device. In one embodiment of the invention, such a compensation network is used to construct a modulator that provides more efficient modulation per unit length of electrode.
Accordingly, the present invention features an electro-optic device that includes an optical waveguide that is formed in an electro-optic material such as lithium niobate. The optical waveguide propagates an optical signal along a first direction of propagation. An electrical waveguide is also formed in the electro-optic material and is positioned co-linear relative to the optical waveguide and in electromagnetic communication with the optical waveguide. The electrical waveguide also propagates an electrical signal in the first direction of propagation.
In addition, the electro-optic device includes a compensation network that is electrically coupled to the electrical waveguide at a junction. The compensation network propagates the electrical signal in a second direction of propagation that is substantially non-co-linear with the first direction of propagation. In one embodiment, the compensation network includes at least one of an all-pass electrical network, an inductor-capacitor xe2x80x9cPixe2x80x9d network, traveling wave coupler, filter, and transmission line transformer.
The compensation network is designed to modify at least one of the phase or the amplitude of the electrical signal at the junction relative to the phase or the amplitude of the accumulated modulation on the optical signal at the junction, respectively, and then return the modified electrical signal to the electrical waveguide. The compensation network may be a time delay network or a phase delay network.
In one embodiment, the compensation network is a phase delay network that modifies the phase of the electrical signal so that an electro-optic response of the device is increased. In another embodiment, the compensation network is a phase delay network that modifies the phase of the electrical signal so that an electro-optic phase at the junction is substantially equal to an electro-optic phase at an input of the electrical waveguide.
In yet another embodiment, the compensation network is a phase delay network that modifies the phase of the electrical signal at the junction relative to the phase of the accumulated modulation on the optical signal at the junction by a predetermined delay that is variable over a range from zero to one hundred and eighty degrees. In this embodiment, the phase of the electrical signal at the junction relative to the phase of the accumulated modulation on the optical signal at the junction may be modified to be substantially one hundred and eighty degrees.
One advantage of the compensation network of the present invention is that the electrical loss per unit length can be designed to be significantly lower than the electrical loss per unit length of the electrical waveguide to minimize RF losses. Another advantage of the compensation network is that it may be removably attached to the electro-optic device so that it can be replaced by another compensation network with different characteristics. Another advantage of the compensation network is the temperature dependence of the compensation network can be made to be inversely proportional to the temperature dependence of the electro-optic material so as to compensate for temperature non-linearity in the electro-optic material.
The present invention also features an electro-optic modulator that includes a plurality of compensation networks. The optical waveguide is formed in an electro-optic material such as lithium niobate. The optical waveguide propagates an optical signal along a first direction of propagation. An electrical waveguide is formed in the electro-optic material and is positioned in a co-linear direction relative to the optical waveguide and in electromagnetic communication with the optical waveguide. The electrical waveguide also propagates an electrical signal in the first direction of propagation. Each of the plurality of compensation networks are electrically coupled to the electrical waveguide at one of a plurality of junctions. Each of the compensation networks propagates the electrical signal in a second direction of propagation that is substantially non-co-linear with the first direction of propagation.
In operation, each of the plurality of compensation networks modifies a phase of the electrical signal at a respective junction of the plurality of junctions relative to a phase of the accumulated modulation on the optical signal at the respective junction by a predetermined delay and then returns the modified electrical signal to the electrical waveguide. The predetermined delay is variable over a range from zero to one hundred and eighty degrees and, in one embodiment of the invention, the predetermined delay is substantially one hundred and eighty degrees. In another embodiment, each compensation network modifies the phase of the electrical signal at the respective junction relative to the phase of the accumulated modulation on the optical signal at the respective junction so that an electro-optic response of the device is increased.